There are several techniques used to measure stress levels, such as arterial blood pressure or inflatable cuffs, but they are often invasive, cumbersome, and inaccurate. However, measurements of cardiac activity are relatively non-invasive and reliable. Heart rate monitors are known to readily detect heart rate from various points in the body through the use of a pulse oximeter.
Heart rate variability is one of several known physiological marks of stress. Heart rate variability is the variation of the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval. Variation in the beat-to-beat interval is a physiological phenomenon that involves inputs to the sinoatrial (SA) node of the heart from the two branches of the autonomic nervous system: (1) the sympathetic nervous system (SNS) and (2) the parasympathetic nervous system (PNS). The SNS is responsible for triggering the stress inducing, fight or flight response in the body and typically has a sluggish activation. The PNS counters the SNS response by bringing the body back towards relaxation and rests states and typically has a rapid activation.
SNS activation increases heart rate, while PNS activation decreases heart rate. Analysis of heart rate variability allows for deciphering between the contributions made by the PNS and the SNS, respectively. Several techniques have been used to attempt to decipher the SNS and PNS contributions on heart rate variability. One such technique for estimating PNS and SNS activation is through principal dynamic modes. Principal dynamic modes have been used to detect mentally stressful events using heart rate monitors or ECG monitors.
Prior attempts to measure beat-to-beat values and the relationship and activity between the PNS and the SNS have used the following equations:
                              y          ^                PNS            ⁡              (        t        )              =                  ∑                              λ            i                    >          0                                              ⁢                          ⁢                                    λ            i                    ⁡                      (                                                            v                  i                                *                                  X                  ⁡                                      (                    t                    )                                                              +                              μ                                  i                  ,                  0                                                      )                                                             *                    ⁢          2                                                  y          ^                SNS            ⁡              (        t        )              =                  ∑                              λ            i                    <          0                                              ⁢                          ⁢                                    λ            i                    ⁡                      (                                                            v                  i                                *                                  X                  ⁡                                      (                    t                    )                                                              +                              μ                                  i                  ,                  0                                                      )                                               2                    
Previous attempts used to map various dynamic modes into SNS and PNS influences appear to be experimental and have not yet been verified. Thus, there is no reliable technique for measuring the relationship between the SNS and the PNS and then using that relationship to improve the mental health levels of individuals.
Software applications exist for tracking user movements, monitoring user heart rates, and providing feedback to users regarding their heart rates. However, there do not appear to be any systems, methods, or applications for detecting and identifying the stress levels of an individual by measuring the individual's heart rate variability. Thus, there exists a need to accurately distinguish between the SNS and the PNS in order to better monitor and track the stress levels of an individual. There exists a need to provide accurate feedback to users regarding their stress levels over an extended period of time for improved mental health states.